You can't use frozen semen for stud purposes. The owners are doing this because they love the horse. If they wanted money, they could have "put him down" at the track for the insurance money.
While it's true that horse semen is much more fragile than bull semen, equine artificial insemination has become the norm. It is cheaper to hire a vet to collect the semen and freeze it than the transportation costs involved in shipping a mare across country.